In the field of optical communications, especially at high bit rates, phase modulation has many advantages, both from the viewpoint of spectral efficiency and that of transmission quality (because of the reduction of non-linear effects related to the modulation of intensity in particular).
For example, the DBPSK modulation format is particularly advantageous for non-coherent transmission. In direct detection, the phase of the transmitted signal is lost at reception but can be retrieved through measurement of the difference in phase between two successive symbols, through an optical demodulation performed by an MZDI (a Mach-Zender delay interferometer) type interferometer for example. The differential encoding of the data to be transmitted proves to be necessary in order to enable the use of a phase modulation in direct detection.
The QPSK and PDM-QPSK modulation formats for their part are particularly worthwhile for coherent transmissions having a bit rate greater than or equal to 40 Gb/s. The use of a coherent receiver makes the phase of the transmitted signal directly accessible. It is therefore possible to use modulation formats with high spectral efficiency. However, the phase noise remains a major problem which the phase retrieval algorithms (of the Viterbi type) are unable to process totally. Here too, the differential encoding of the data to be transmitted proves to be necessary.
It is thus seen that differential encoding combined with phase modulation improves the performance of the transmission system.
Unfortunately, the implementing of differential encoding leads to a higher error rates than a classic encoding because a transmission error produces two errors on the data transmitted in the form of information symbols. More generally, k successive transmission errors produce k+1 errors on the information symbols.
More specifically, in the case of a QPSK type modulation for example, the classic encoding as illustrated in FIG. 1A enables the association of an information symbol comprising two bits with a modulated symbol corresponding to a state αi of the constellation. For a QPSK modulation, the states of the constellation associated with the modulated symbols are: α1=eiπ/4, α2=e3iπ/4, etc.
Differential encoding as illustrated in FIG. 1B makes it possible, for its part, to encode the data to be transmitted by the transition between the states of the constellation used. In other words, an information symbol corresponds to two bits which encode the change in phase between two states αi of the constellation, i.e. between two modulated symbols. For example, the sequence of information symbols ‘10’ ‘11’ ‘00’ is converted into modulated symbols corresponding to the states α1, α4, α2, α2. If, after passing through the transmission channel, the modulated symbols received correspond to the states α1, α4, α1, α2, which corresponds to one transmission error, then the reconstructed information symbols will be ‘10’ ‘01’ ‘01’, corresponding to two errors on the transmitted data (shown as underlined figures).
The differential encoding leads to higher rates of error than classic encoding because one transmission error corrupts two consecutive information symbols.
There is therefore a need for a novel technique of transmission implementing a differential modulation enabling the transmitted data to be reconstructed more reliably.